Many properties of a subterranean formation may be determined using different oilfield logging techniques, which may involve one or more tools having a radioisotope source. For example, to locate gas in a subterranean formation, a conventional practice combines data obtained from two tools. One of the tools is a “density” tool, which measures the electron density of the formation, and the other of the tools is a “neutron porosity” tool, which generally measures the density of hydrogen in the formation, known as the “hydrogen index (HI).” Based on measurements of formation density and hydrogen index, the porosity and pore fluid density of the formation may be determined. For a given formation fluid density, or gas saturation, a combination of a decrease in the formation density and an increase in the hydrogen index indicates an increase in the porosity of the formation. Meanwhile, for a given formation porosity, a combination of a decrease in the formation density and a decrease in hydrogen index indicates a decrease in the pore fluid density and hydrogen content. For pores filled with water and gas or oil and gas, the density and hydrogen index are an indication of the gas saturation (volume fraction of the pores occupied by gas). For pores filled with gas only, the density and hydrogen index are an indication of gas density (pressure).
The density and neutron porosity tools for measuring formation density and hydrogen index may generally employ radioisotope sources to obtain formation density and hydrogen index measurements, respectively. For example, the density tool may use a source such as 137Cs to emit gamma-rays into a formation. Based on a count of gamma-rays scattered by the formation, the density tool may determine the electron density of the formation. Similarly, the neutron porosity tool may use a source such as 241AmBe to emit neutrons into a formation. A count of neutrons scattered by the formation may yield a hydrogen index measurement. Such radioisotope sources may be disadvantageous in oilfield tools, as the sources may be heavily regulated by law and they can be hazardous since they cannot be shut off.
In lieu of such radioisotope sources, an electronic neutron generator may be used which will produce neutrons which, in turn, produce gamma-rays. To do so, the electronic neutron generator may emit neutrons into a formation, which may in turn produce gamma-rays via inelastic scattering and neutron capture events. A count of gamma-rays produced by inelastic scattering may generally yield a signal that is related to formation density, and a count of scattered neutrons may generally yield a neutron porosity signal that corresponds to the hydrogen index of the formation. Alternatively, a count of capture gamma-rays may generally yield a neutron porosity signal that corresponds to the hydrogen index of the formation. If it is not possible to separate the inelastic and capture gamma-rays to produce nearly independent signals sensitive to formation density and hydrogen index, respectively, then the two signals may not be used together to enable a precise determination of porosity and gas saturation.
Neutron reactions that produce gamma-rays may be separated according to the energy of the neutron. After a 14 MeV neutron has been emitted by the source, it begins to lose energy by the processes of elastic and inelastic scattering. Inelastic scattering events are typically produced by neutrons in the energy range 1-14 MeV. After neutrons have decreased in energy below approximately 1 MeV, they typically have insufficient energy to inelastically scatter; however, they continue to lose energy by elastic scattering. The decrease in energy from 14 MeV to 1 MeV happens very rapidly, in a time typically less than 1 microsecond. Inelastic scattering reactions therefore occur very quickly after the neutron leaves the source, typically in less than 1 microsecond. From approximately 1 MeV down to thermal energy (approximately 0.025 eV), neutrons decrease in energy by elastic scattering over a time interval that ranges from 2 to several microseconds, depending on the amount of hydrogen in the formation. During that slowing time, neutrons may be captured and this may lead to the emission of one or more gamma-rays. These are so-called “epithermal” capture gamma-rays. Neutrons which decrease in energy completely to thermal energy continue to elastically scatter at that energy, often for many hundreds of microseconds until they are captured and this may lead to the emission of one or more gamma-rays. These are so-called “thermal” capture gamma-rays. Since neutrons are emitted from an electronic neutron source typically in bursts no shorter than 10 microseconds, it will be appreciated that the inelastic and epithermal capture gamma-rays are emitted substantially within that 10 microsecond burst and therefore overlap in time. Thermal capture gamma-rays, on the other hand, extend into the time interval between bursts as well as during the burst.
FIG. 1 shows a crossplot of the normalized ratio of true inelastic counts of a far and near gamma ray detector on the y-axis and the normalized ratio of capture counts (40-80 microseconds after the neutron burst), both derived from modeling, where specific neutron and photon interactions of different neutron energy groups were separately tallied, which is not possible with a logging tool. What can be seen from this plot is the independence of the two axes giving a nearly orthogonal relationship between water filled porosity and gas filled porosity. This relationship is similar to the well known open-hole neutron-density log crossplot.
A complication arises due to the interaction of neutrons with hydrogen. After inelastically scattering at MeV energy, neutrons continue their scattering process in the formation, and slow down to eV and sub-eV energies mainly through elastic scattering from hydrogen. The number of low energy neutrons reaching the vicinity of the gamma-ray detector is strongly influenced by the hydrogen index, just as it is in a neutron porosity measurement. If these neutrons are captured by a nucleus in the gamma-ray detector or in the vicinity of the detector and the nucleus subsequently emits a gamma-ray that is detected by the detector, this signal will contaminate the “density” signal coming from inelastic scattering events. Indeed, it is possible for capture events to completely overwhelm the inelastic events, leading to a gamma-ray detector response which has the character of a neutron porosity measurement rather than a density measurement. It is fruitless to combine such a measurement with a neutron porosity measurement to try to identify gas because the two measurements are not independent of one another.
FIG. 2 shows a similar crossplot to FIG. 1 where on the y-axis is the normalized ratio of total counts during the initial 3 microseconds of the neutron burst and on the x-axis is the normalized ratio of capture counts as in FIG. 1. Note that the orthogonality of the two axes that is apparent in FIG. 1 no longer exists. Both axes have a significant hydrogen index response which masks the density response of the inelastic ratio, so that the measurement does not distinguish higher gas-filled porosity from lower water-filled porosity.
Data on cased hole “neutron” density measurements can be found in various SPE papers (see, e.g., SPE 71042, SPE 71742, SPE 56647, SPE 55641, and SPE 38699), but the discussion is limited to neutron capture background in terms of thermal neutron capture, without handling epithermal neutron capture. US patent application PCT/US10/35718, commonly assigned and incorporated herein by reference in its entirety, discusses design features to minimize epithermal background.